Anatase-type titanium oxide powder and method for producing same

ABSTRACT

An anatase-type titanium oxide powder having a ratio of rutile to anatase of 10% or less and a BET specific surface area of 20 to 80 m 2 /g. Since the titanium oxide powder has a large specific surface area and a low ratio of rutile to anatase in comparison with a conventional titanium oxide powder and excels in dispersibility, the titanium oxide powder is suitable for various applications.

TECHNICAL FIELD

The present invention relates to anatase-type titanium oxide powder witha small specific surface area and a low ratio of rutile to anatase, andparticularly to titanium oxide which can be used for a photocatalyst, anoptical material, and the like.

BACKGROUND ART

Titanium oxide powder has been used as a white pigment for many years.More recently, titanium oxide powder is widely used as a UV shieldingmaterial for cosmetics and the like, a material for forming aphotocatalyst, a capacitor, or a thermistor, and a sintered materialused for electronic materials such as a raw material of barium titanate.Due to possession of a large refractive index in a wavelength regionnear visible light, the titanium oxide absorbs almost no light in thevisible-light region. For these reasons, titanium oxide powder isrecently used as a material that requires UV shielding properties suchas a cosmetic composition, drug, or coating material and as anantireflection film for a display part of a liquid crystal display and aplastic lens. The antireflection film typically comprises a layer formedby alternately laminating a layer of a resin with a low refractiveindex, such as a fluororesin or silicon-containing resin, and a highrefractive index layer. Titanium oxide is used as a material for thehigh refractive index layer of the antireflection film. In addition, inplasma displays for which demand is recently increasing, a glassmaterial used for substrate partitions is covered with titanium oxide inorder to promote brightness and improve reflectance or is caused tocontain rutile-type titanium oxide powder in order to improve therefractive index.

The rutile-type titanium oxide is more excellent than anatase-typetitanium oxide in optical characteristics, such as UV shieldingcharacteristics and high refractive index, and electrical propertiessuch as high dielectric properties.

Titanium oxide is excited when irradiated with light having energy notless than its band gap and produces electrons in the conduction band andpositive holes in the valence band. Development for application of aphotocatalyst utilizing a reduction power of the electrons and anoxidation power of positive holes is actively undertaken. There arevarious applications of the titanium oxide photocatalyst. A number ofapplication developments such as hydrogen production by decomposition ofwater, exhaust gas treatment, air cleaning, deodorization,sterilization, antibacterial treatment, waste water treatment,stain-proofing of illumination equipment, and the like are ongoing.

The anatase-type titanium oxide is used as a photocatalyst material dueto the high catalytic activity.

As a method for obtaining anatase-type titanium oxide, Japanese PatentApplication Laid-open No. 8-333117 discloses a method for producinganatase-type titanium oxide particles having a uniform particle size anda large specific surface area. The method comprises preparing an aqueoussolution of titanyl sulfate containing titanyl sulfate in an amount of5.0-100 g/l (as TiO₂) and an excess amount of sulfuric acid of 1.0-3.0times of Ti (as mol), adding urea in an amount equimolar or more to thetotal sulfate group, heating the mixture at a temperature from 85° C. toboiling point to collect deposited meta-titanic acid particles, andbaking the meta-titanic acid particles at 650-850° C.

Japanese Patent Application Laid-open No. 2001-287997 discloses a methodof oxidizing titanium tetrachloride in a gas phase oxidation reaction bycausing 1 l titanium tetrachloride gas to come in contact with 1-30 l ofoxygen (gas volume being indicated in a standard state gas volume) at700-850° C. and to produce titanium oxide particles and heat-treatingthe resulting titanium oxide particles at 300-850° C.

-   (Patent Document 1) Japanese Patent Application Laid-open No.    8-333117-   (Patent Document 2) Japanese Patent Application Laid-open No.    2001-287997

The titanium oxide particles obtained by the method disclosed by PatentDocument 1 have a large specific surface area and exhibit only poordispersibility when used as a photocatalyst. In addition, handling ofthe titanium oxide particles is very difficult. The titanium oxidepowder disclosed by Patent Document 2 has a high ratio of rutile toanatase of 20%.

These titanium oxide powders are mixtures of rutile-type titanium oxideand anatase-type titanium oxide. Even though their specific surface areais small and dispersibility is excellent, their ratio of rutile toanatase is comparatively high. Titanium oxide with a smaller ratio ofrutile to anatase has a large specific surface area and exhibits onlypoor dispersibility. Therefore, titanium oxide powder exhibiting higherphotocatalyst activity and excellent dispersibility has been demanded.

An object of the present invention is therefore to provide titaniumoxide powder having a specific surface area larger than the specificsurface area of the titanium oxide powder described in Patent Document 2and having a ratio of rutile to anatase smaller than the ratio of rutileto anatase of the titanium oxide powder described in Patent Document 2,and a method for producing the titanium oxide powder.

DISCLOSURE OF THE INVENTION

As a result of extensive studies to achieve the above object, theinventor of the present invention has found an anatase-type titaniumoxide powder having a larger specific surface area and a smaller ratioof rutile to anatase than those of the titanium oxide powder describedin Patent Document 2. This finding has led to the completion of thepresent invention.

Specifically, the titanium oxide powder of the present invention has aratio of rutile to anatase of 10% or less and a BET specific surfacearea of 20 to 80 m²/g.

In addition, the inventor has discovered a method for producing theanatase-type titanium oxide powder having a ratio of rutile to anataseof 10% or less and a BET specific surface area of 20 to 80 m²/g by gasphase oxidation or gas phase hydrolysis of titanium tetrachloride,leading to completion of the present invention.

Specifically, the present invention provides a method for producing ananatase-type titanium oxide powder with a small specific surface areacomprising reacting titanium tetrachloride gas, oxygen gas, hydrogengas, and steam in a gas phase, characterized by previously heating eachraw material gas of the titanium tetrachloride gas, oxygen gas, hydrogengas, and steam at 450 to 650° C., and feeding the raw material gases ata rate of 60 to 90 l of oxygen gas, 60 to 90 l of hydrogen gas, and 240to 600 l of steam for 1 l of titanium tetrachloride gas.

The titanium oxide powder of the present invention has a small BETspecific surface area in spite of a small ratio of rutile to anatase andis useful as a photocatalyst. The method for producing titanium oxidepowder of the present invention can produce an anatase-type titaniumoxide powder having a small BET specific surface area in spite of asmall ratio of rutile to anatase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart showing results of dispersibility evaluation by adynamic light scattering method.

BEST MODE FOR CARRYING OUT THE INVENTION

The ratio of rutile to anatase of the titanium oxide powder of thepresent invention is 10% or less, preferably 8% or less, and still morepreferably 5% or less. The low ratio of rutile to anatase in this rangeensures a high photocatalyst activity.

The ratio of rutile to anatase is measured by X-ray diffractionaccording to the method of ASTM D3720-84, in which the peak area (Ir) ofthe strongest diffraction line (index of plane 110) of rutile-typecrystal titanium oxide and the peak area (Ia) of the strongestdiffraction line (index of plane 101) of anatase-type crystal titaniumoxide are measured and applied to the following formula.

Ratio of rutile to anatase (wt %)=100−100/(1+1.2 ×Ir/Ia)

Ir and Ia refer to the areas projecting from the baseline in theapplicable diffraction line of the X-ray diffraction pattern. Theseareas are determined by a known method such as computer calculation, anapproximation triangle-formation method, or the like.

The BET specific surface area of the titanium oxide powder of thepresent invention is from 20 to 80 m²/g, preferably from 20 to 60 m²/g,and still more preferably from 20 to 40 m²/g. The average particlediameter is from 10 to 100 nm, preferably from 15 to 80 nm, and stillmore preferably from 20 to 75 nm.

Because the anatase-type titanium oxide powder of the present inventionis obtained by a gas phase method of oxidizing titanium tetrachloride bya gas phase oxidation reaction, the titanium oxide powder has no problemof residual or mingled impurities as in the case of titanium oxidepowder obtained by a liquid phase method. In particular, the content ofFe, Al, Si, and Na is respectively less than 100 ppm and the content ofCl is 700 ppm or less, and the content of S is less than 10 ppm,preferably less than 7 ppm, and particularly preferably less than 5 ppm.

As the method for producing anatase-type titanium oxide of the presentinvention, a gas phase reaction method of oxidizing titaniumtetrachloride by causing the titanium tetrachloride to come into contactwith oxygen in a gas phase, a flame hydrolyzing method comprisingsupplying an inflammable gas such as hydrogen gas, which generates waterwhen burned, together with oxygen to a combustion burner, therebyforming a flame, and introducing titanium tetrachloride into the flame,and the like can be given.

The method for producing titanium oxide powder of the present inventionwill now be described in detail.

The method for producing titanium oxide of the present invention is amethod of hydrolyzing or oxidizing titanium tetrachloride in a gas phaseand specifically comprises causing titanium tetrachloride vapor to comein contact with hydrogen gas, oxygen gas, and steam in a gaseous state.The amount of oxygen gas and hydrogen gas to be supplied is 60 to 90l,preferably 70 to 90l , and more preferably 80 to 90 lfor 1 l of titaniumtetrachloride gas supplied to a reaction section. The amount of steam tobe supplied is 240 to 600 l for 1 l of titanium tetrachloride gassupplied to the reaction section. If the amount of steam is too large,hydrolysis of titanium tetrachloride is promoted, resulting inproduction of fine titanium oxide particles with an increased specificsurface area. If the amount of steam is too small, a sufficient reactionwill not occur, resulting in a titanium oxide with unreacted titaniumtetrachloride mixed therein. In order to produce titanium oxide with aspecific surface area of 20 to 40 m²/g, the amount of steam supplied ispreferably 240 to 400l . The amount of each raw material gas suppliedvaries according to the scale of reaction, the diameters of the nozzlesused for supplying each gas, and the like and, therefore, a specificamount of each raw material gas is appropriately determined. The feedrate of each gas in the reaction section, particularly the feed rate oftitanium tetrachloride gas, is preferably set so that a turbulent flowregion may be formed in a combustion flame of the reaction section. Inaddition, each component gas may be supplied to and reacted in thereaction section after being diluted with an inert gas such as argon ornitrogen.

The titanium tetrachloride gas, oxygen gas, hydrogen gas, and steam aresupplied to the reaction section after preheating at 450 to 650° C., andpreferably to 500 to 600° C. If the preheating temperature is too high,the ratio of rutile to anatase will increase; if less than 450° C., thereaction does not sufficiently proceed.

After producing titanium oxide powder by the reaction of the gases inthe above manner, the reaction system is cooled to a temperature nothigher than the temperature at which titanium oxide particles aresintered, specifically to a temperature of about 200° C. in order toprevent aggregation of produced particles.

After removing chlorine components such as hydrogen chloride by heattreatment or the like, the titanium oxide powder obtained in this manneris classified or sieved, as required, thereby obtaining anatase-typetitanium oxide powder of the present invention.

A specific process for producing anatase-type titanium oxide powder ofthe present invention will now be described. First, liquid titaniumtetrachloride is vaporized by preheating at 450 to 650° C. and, asrequired, diluted with nitrogen gas before being introduced to thereactor. Simultaneously with the introduction of titanium tetrachloride,oxygen gas, hydrogen gas, and steam preheated at 450 to 650° C., afterdilution with nitrogen gas, as required, are introduced to the reactor,wherein the oxidation reaction is carried out. The oxidation reactionmust be carried out at a temperature not lower than the temperature atwhich titanium oxide is produced and not higher than the temperature atwhich the crystal structure of the titanium oxide is converted into arutile type, usually in a range from 500 to 700° C., and preferably from550 to 650° C. Oxidation at a comparatively low temperature is preferredfor obtaining the anatase-type titanium oxide powder of the presentinvention.

The produced anatase-type titanium oxide particles are introduced into acooling section to cause the titanium oxide powder to come into contactwith a coolant gas such as air, thereby cooling the titanium oxidepowder to about 200° C. After that, the produced titanium oxide powderis collected to remove chlorine components remaining in the titaniumoxide powder by means of a heat treatment such as heating under vacuum,heating in an air or nitrogen gas atmosphere, or steam processing or bycontact with alcohol, thereby obtaining the anatase-type titanium oxidepowder of the present invention. To remove chlorine components remainingin the titanium oxide powder by a heat treatment in the air, thetitanium oxide powder is heated at 300 to 400° C. for 10 to 20 hours.

The anatase-type titanium oxide powder of the present invention isuseful as a photocatalyst, an optical material, and the like,particularly as a photocatalyst.

The present invention will be described in more detail by examples,which should not be construed as limiting the present invention.

EXAMPLE 1

Anatase-type titanium oxide powder was produced by the gas phasereaction method of oxidizing titanium tetrachloride by causing it tocome into contact with oxygen in a gas phase. Using a gas phase reactionpipe equipped with a multiplex pipe burner, with an inner diameter of 12mm and outer diameter of 20 mm, on the top, titanium tetrachloride gaspreheated at about 500° C. was supplied to the multiplex pipe burner,while supplying hydrogen gas, oxygen gas, and steam preheated at 500° C.from another nozzle, thereby effecting the oxidation reaction in the gasphase reaction pipe at about 600° C. to produce titanium oxide powder.The feed rate of titanium tetrachloride, oxygen gas, hydrogen gas, andsteam under the standard state was respectively 500 ml/min, 40 l/min, 40l/min, and 130 l/min. After that, air at room temperature was suppliedto the cooling section in the lower part of the gas phase reaction pipeat a rate of 400 l/min to cool the produced titanium oxide powder. Theresulting titanium oxide powder was treated with heat at 380° C. in theatmosphere for 18 hours. The average particle diameter, ratio of rutileto anatase, specific surface area, content of impurities, and particlesize distribution of the resulting titanium oxide powder were measured.The results are shown in Table 1. The average particle diameter, ratioof rutile to anatase, specific surface area, content of impurities, andparticle size distribution of the titanium oxide powder were measuredaccording to the following methods.

(Average Particle Diameter)

The titanium oxide powder was inspected using a scanning electronmicroscope to measure the average particle diameter by the interceptmethod. (Number of analyzed particles: 200)

(Ratio of Rutile to Anatase)

The ratio of rutile to anatase was determined by measuring the X-raydiffraction pattern according to the method of ASTM D 3720-84, in whichthe peak area (Ir) of the strongest interference line (index of plane110) of rutile-type crystal titanium oxide and the peak area (Ia) of thestrongest interference line (index of plane 101) of the anatase-typecrystal titanium oxide were measured and applying the results to theabove-described formula. The X-ray diffraction measurement conditionswere as follows.

-   (X-ray diffraction measurement conditions)-   Instrument: RAD-1C (manufactured by Rigaku Corp.)-   X-ray tube ball: Cu-   Tube voltage and tube current: 40 kV, 30 mA-   Slit: DS-SS: 1°, RS: 0.15 mm-   Monochrometer: graphite-   Measurement interval: 0.002°-   Counting method: Scheduled counting method    (Specific Surface Area)

Measured by the BET method.

(Impurity Content)

Sulfur components in the titanium oxide were measured by the combustioninfrared absorption method. Fe, Al, Si, and Na in titanium oxide weremeasured by plasma emission spectral analysis. Cl in titanium oxide wasmeasured by titration used silver nitrate.

(Particle Size Distribution)

Using a laser scattering diffraction particle size analyzer (LA-700:Horiba, Ltd.), an appropriate amount of titanium oxide powder wassuspended in purified water, followed by dispersing the particles byapplication of ultrasonic wave for three minutes. The particle size wasmeasured and the particle size distribution (volume statistic value) wasdetermined. The particle size distribution (SPAN) was determined fromD90 (90% particle size (μm) in cumulative particle size), D50 (50%particle size (μm) in cumulative particle size), and D10 (10% particlesize (μm) in cumulative particle size) according to the followingformula.SPAN=(D90−D10)/50

EXAMPLE 2

Titanium oxide powder was prepared in the same manner as in Example 1except for supplying steam at a rate of 200 l/min. The average particlediameter, ratio of rutile to anatase, specific surface area, content ofimpurities, and particle size distribution of the resulting titaniumoxide powder are shown in Table 1.

EXAMPLE 3

Titanium oxide powder was prepared in the same manner as in Example 1except for supplying steam at a rate of 300 1/min. The average particlediameter, ratio of rutile to anatase, specific surface area, content ofimpurities, and particle size distribution of the resulting titaniumoxide powder are shown in Table 1.

COMPARATIVE EXAMPLE 1

Titanium oxide powder was prepared in the same manner as in Example 1except for supplying oxygen gas and hydrogen gas at a rate of 95 1/minand supplying steam at a rate of 350 1/min. The average particlediameter, ratio of rutile to anatase, specific surface area, content ofimpurities, and particle size distribution of the resulting titaniumoxide powder are shown in Table 1.

COMPARATIVE EXAMPLE 2

Titanium oxide powder was prepared in the same manner as in Example 1except for preheating titanium tetrachloride gas, hydrogen gas, oxygengas, and steam at 800° C. The particle diameter, ratio of rutile toanatase, specific surface area, content of impurities, and particle sizedistribution of the resulting titanium oxide powder were measured. Theresults are shown in Table 1.

COMPARATIVE EXAMPLE 3

An experiment for producing titanium oxide powder was carried out in thesame manner as in Example 1 except for preheating titaniumtetrachloride, hydrogen gas, oxygen gas, and steam at 400° C. andsupplying hydrogen gas and oxygen gas at a rate of 20 l/min andsupplying steam at a rate of 110 1/min. However, the reaction did notproceed and titanium oxide powder was not produced. TABLE 1 ExampleComparative Example 1 2 3 1 2 Average particle 70 50 40 12 50 diameter(nm) Ratio of rutile 4.6 2.2 4.2 8.6 92.2 to anatase (%) Specificsurface 26.0 33.3 42.4 86.2 30.5 area (m²/g) Impurities content (ppm) S<10 <10 <10 <10 <10 Fe 10 10 10 10 10 Al <10 <10 <10 <10 <10 Si <10 <10<10 <10 <10 Na <10 <10 <10 <10 <10 Cl 500 570 690 820 540 Particle sizedistribution D90 1.80 1.18 2.08 2.65 1.85 D50 0.60 0.44 0.41 0.40 0.54D10 0.18 0.15 0.14 0.10 0.20 SPAN 2.7 4.9 4.7 6.4 3.1

As is clearly shown in Table 1, the titanium oxide powders obtained inthe experiments of the Examples, in which titanium tetrachloride gas,oxygen gas, hydrogen gas, and steam were preheated at 450 to 650° C.,oxygen gas and hydrogen gas were supplied at a rate of 60 to 90 l/min,and steam was supplied at a rate of 240 to 600 l/min for the amount oftitanium tetrachloride gas of 1 l/min, had a low ratio of rutile toanatase of 10% or less and a small specific surface area as comparedwith the titanium oxide powder obtained in Comparative Example 1. Inaddition, the titanium oxide powders of the Examples had a large averageparticle diameter and narrow particle size distribution. These titaniumoxide powders exhibited excellent dispersibility in solvents. Thetitanium oxide powder of Comparative Example 1 had a large specificsurface area due to a large supply amount of steam. In addition, due toan excess supply amount of oxygen gas and hydrogen gas, the titaniumoxide powder exhibited poor particle size distribution and poordispersibility. In Comparative Example 2, the titanium oxide powder hada high ratio of rutile to anatase due to a very high preheatingtemperature of titanium tetrachloride, oxygen gas, hydrogen gas, andsteam. In Comparative Example 3, due to a low preheating temperature andsmall supply amounts of oxygen gas, hydrogen gas, and steam, theoxidation reaction of titanium tetrachloride did not sufficientlyproceed and anatase-type titanium oxide powder could not be obtained.

(Evaluation of Dispersibility by Dynamic Light Scattering Method)

Dispersibility of the titanium oxide powders of Example 1 andComparative Example 2 was evaluated by the dynamic light scatteringmethod. In the evaluation of dispersibility by the dynamic lightscattering method, the colloidal particle diameter of titanium oxidepowder in water versus distribution time was measured using “N5”(manufactured by Beckmann Coulter Co., Ltd.). The titanium oxide powderconcentration was 10 wt %. The results are shown in FIG. 1. As can beseen in FIG. 1, the titanium oxide powder of the present inventionexhibits excellent dispersibility, that is, the titanium oxide particleswere efficiently dispersed in a short time.

(Evaluation as Barium Titanate Powder)

Barium titanate powders were prepared using the titanium oxide powdersof Examples 2 and 3 and Comparative Example 2 to measure the specificsurface areas and average particle diameters. 10 g of titanium oxidepowders of Examples 2 and 3 and Comparative Example 2 and 10 g of bariumcarbonate powder with a specific surface area of 10 m²/g were mixed andground for 24 hours by ball milling with zirconia balls with a diameterof 1.5 mm. Specific surface areas of mixed particles after ball millingare shown in Table 2. The ground fine particles were sintered at 900° C.for two hours to obtain barium titanate powders. The average particlediameters of the resulting barium titanate powders are shown in Table 2.TABLE 2 Titanium oxide Titanium oxide Titanium oxide powder of powder ofpowder of Comparative Example 2 Example 3 Example 2 Specific surfacearea 22.0 21.1 18.0 (m²/g) Average particle 100 101 200 diameter (nm)

It can be seen from Table 2 that when titanium oxide powder of thepresent invention with excellent dispersibility is used, mixed powderswith a large specific surface area can be obtained by mixing andgrinding by ball milling. In addition, it can be seen that the bariumtitanate powder obtained by sintering the mixed powder has a smallaverage particle diameter and is an optimal material particularly forminiaturization of laminated ceramic capacitors.

INDUSTRIAL APPLICABILITY

The anatase-type titanium oxide powder of the present invention isuseful as a photocatalyst. In addition, since the titanium oxide powderexcels in dispersibility, barium titanate powder obtained therefrom hasa small average particle diameter and can be applied particularly tominiaturization of multilayer ceramic capacitors.

1. An anatase-type titanium oxide powder having a ratio of rutile toanatase of 10% or less and a BET specific surface area of 20 to 80 m²/g.2. The anatase-type titanium oxide powder according to claim 1, obtainedby gaseous phase reaction of titanium tetrachloride.
 3. The anatase-typetitanium oxide powder according to claim 1, obtained by reactingtitanium tetrachloride, oxygen gas, hydrogen gas, and steam in a gaseousphase.
 4. The anatase-type titanium oxide powder according to claim 1,obtained by preheating titanium tetrachloride, oxygen gas, hydrogen gas,and steam at 450 to 650° C. and reacting them in a gaseous phase.
 5. Theanatase-type titanium oxide powder according to claim 1, wherein theamounts of oxygen gas, hydrogen gas, and steam supplied are respectively60 to 90 1, 60 to 90 1, and 240 to 600 1 per 1 1 of titaniumtetrachloride gas.
 6. The anatase-type titanium oxide powder accordingto claim 1, having a sulfur atom content of less than 10 ppm.
 7. Theanatase-type titanium oxide powder according to claim 1, having anaverage particle diameter of 10 to 100 nm.
 8. A method for producinganatase-type titanium oxide powder comprising preheating titaniumtetrachloride, oxygen gas, hydrogen gas, and steam at 450 to 650° C. andreacting them in a gaseous phase.
 9. The method for producinganatase-type titanium oxide powder according to claim 8, wherein theamounts of oxygen gas, hydrogen gas, and steam supplied are respectively60 to 90 1, 60 to 90 1, and 240 to 600 1 per 1 1 of titaniumtetrachloride gas.